The present disclosure relates to a measuring system having a measuring tool which comprises a probe body and an optical marker, having a camera for recording image data of the measuring tool, and having an evaluation and control unit which is configured to evaluate the image data recorded by the camera and to use said data to determine spatial position coordinates of the optical marker.
The present disclosure further relates to a corresponding measuring method as well as to a computer program product, which comprises a program code that is configured to perform the method when it is run on a computer for controlling the aforementioned measuring system.
An exemplary measuring system of the above-mentioned general type is known from DE 10 2015 205 615 A1.
Measuring systems of this type serve to check workpieces, for example within the scope of quality assurance, or ascertain the geometry of a workpiece completely within the scope of what is known as “reverse engineering”. Moreover, diverse further application possibilities are conceivable, such as e.g. process-controlling applications, in which the measurement technique is applied directly for the online monitoring and control of manufacturing and processing processes. A common application example is that of checking vehicle body components in respect of possible manufacturing faults. In principle, however, such measuring systems can be used to measure any type of measurement objects.
Measuring systems having handheld measuring tools are used as an alternative to more complicated coordinate measuring machines in which the workpieces are measured either optically and/or in a tactile manner on a stationary or permanently installed machine with a relatively complex structure.
Due to the mobile usability, measuring systems having handheld measuring tools are becoming increasingly important because they have the potential to extend the range of uses yet further in comparison with stationary or permanently installed coordinate measuring machines solely due to their flexible usability. However, the extremely stringent requirements for the measurement accuracy that these measuring systems are intended to deliver often count against the usability of such a mobile measuring system. It is true that manifold digital-optical possibilities now exist, in particular software methods, in order that, from images or films of objects or scenes, the spatial structure of the imaged objects in the scene may be deduced. In principle, these methods have some shortcomings, however, which result in them currently not yet being suitable for many highly precise measurements but only being used for measurements which have lower requirements for the measurement accuracy.
In the measuring system known from DE 10 2015 205 615 A1, a tactile probe head, which can be used to manually scan a workpiece to be measured, is arranged on a manually portable measuring tool. Furthermore, a plurality of optical markers are arranged on a handle of the measuring tool and regularly emit infrared beams which are captured from the outside using a camera system. The camera images recorded by the camera system are evaluated in a computing unit, the position and orientation of the markers in space being calculated by means of a suitable computing algorithm. This is usually carried out using optical triangulation methods. The location and position of the probe head and of the probe body relative to the markers can be determined by means of a calibration step. If a user guides the measuring tool towards a workpiece with his hand, with the result that the probe body touches the workpiece, a measuring point on the workpiece can therefore be determined. The shape and location of the workpiece relative to the camera system ultimately result from a suitable multiplicity of such measuring points.
However, the measuring system known from DE 10 2015 205 615 A1 has at least two important disadvantages. On the one hand, active infrared light sources are used as markers. Such active markers which are integrated in the handheld measuring tool have the disadvantage that, due to the development of heat caused by them, they give rise to material expansions which may result in measurement errors. Such measurement errors cannot be disregarded at all in optical measurement technology. On the other hand, in the system known from DE 10 2015 205 615 A1, the user must manually actuate a button on an actuation unit in order to signal to the computing unit that a measuring point is intended to be captured or in order to store a currently captured measuring point. Since the user inevitably exerts a force for this purpose, the magnitude and direction of which are unknown, the probe head can be readily deformed, shaken or shifted. Further deformations may occur due to the force of the weight acting on the measuring tool or due to forces of inertia or forces of contact that act on the probe body. This results in measurement errors that cannot be readily compensated.
Similar problems also arise in a system sold by Optinav under the name “OptiTrace” (http://optinay.pl/en/info/products/optitrace.html, retrieved on 22 Dec. 2015).
In stationary or permanently installed coordinate measuring machines, the aforementioned problems are often solved by means of additional sensors that are integrated in the probe head of the coordinate measuring machine. An example of such a measuring system is known from WO 2006/114627 A1. In this case, the probe body or the measuring tip is coupled to the quill of the coordinate measuring machine via springs of the probe head. The movement of the probe head relative to the quill is determined using a separate measuring system. Such probe heads are also referred to as passively measuring sensors for coordinate measuring machines. Another example of a similar system is known from WO 2013/007285 A1.
A similar measuring probe having a load sensor, which is integrated therein, measures the force acting between the probe body and the workpiece and controls the measurement recording based on the signal generated by the load sensor, is known from EP 1 984 695 B1. Although such sensors could also be used in handheld measuring systems, this would considerably increase the overall complexity of the measuring system. In particular, as a result of the additional sensors, further active components would be accommodated in the measuring system, with the result that temporal synchronization of the sensor signals with the signals from the optical tracking system would be required.
Another example of a handheld coordinate measuring machine is known from EP 0 703 517 B1. Apart from the relatively high degree of complexity of this system, the mobile usability is restricted here by the fact that the probe head is connected to a fixed column via a movably mounted carrier.
EP 2 172 735 B1, DE 10 2008 049 751 A1 and DE 100 66 470 B4 also discloses a number of methods for use in stationary or permanently installed coordinate measuring machines in which it is attempted by suitable calibration to compensate for the deflection of the probe body of the coordinate measuring machine that is brought about by the force of the weight, the measuring force or other external forces with the aid of a corresponding control or adjustment of the position of the probe body. However, in the existing form, these methods are only known and suitable for stationary or permanently installed coordinate measuring machines. Corresponding solutions for handheld measuring systems so far do not exist.